Treatment/removal of byproduct aqueous effluents comprising hydroxynitro-aromatic compounds

ABSTRACT

Objectionable byproduct aqueous effluents containing contaminating amounts of hydroxynitroaromatic compounds, in particular those aqueous effluents produced during the synthesis of nitroaromatic compounds, e.g., dinitrotoluenes, via reaction of an aromatic compound with nitric acid in the presence of sulfuric acid, are efficiently, facilely and economically treated/removed by (a) intimately contacting a mixture of at least one nitroaromatic compound and at least one hydroxynitroaromatic compound with an aqueous wash medium containing a neutralizing agent, (b) separating the resulting admixture into an organic phase and an aqueous phase, (c) recycling a fraction of the separated aqueous phase to the aqueous wash medium to thus constitute a portion thereof, and (d) periodically draining a fraction of the wash medium, whether to destruction thereof or to waste.

This application is a continuation, of application Ser. No. 08/676,446,filed Jul. 8, 1996, now abandoned, which is a continuation of 08/367,873filed Jan. 3, 1995. now U.S. Pat. No. 5,554,299.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the elimination of by product aqueouseffluents comprising hydroxynitroaromatic type compounds, and, moreespecially, to the removal of byproduct aqueous effluents which resultduring the preparation and purification of nitroaromatic compounds viareaction of aromatic compounds with nitric acid in the presence ofsulfuric acid.

2. Description of the Prior Art

It is known to this art that, following nitration of aromatic compounds,the reaction medium is treated to separate the desired nitroaromaticcompounds thus produced from the mineral acids employed as reactants andas catalyst, then from the byproducts formed.

This operation normally includes a number of successive wash steps, eachfollowed by a decantation/separation cycle for the organic and aqueousphases. Thus, the reaction medium is first contacted with water torecover the nitric and sulfuric acids. One or more washes are thencarried out using water containing a neutralizing agent to eliminate thebyproducts.

By the term "byproducts" are intended hydroxyaromatic compoundscontaining one, two or three nitro groups (such as nitrophenols andnitrocresols). These compounds are the most objectionable, as they areknown to poison the catalysts used in the final hydrogenation reactionto which the nitroaromatic compounds may be subjected. This reactionreduces the nitro groups to amine groups and the resulting amines canthen be used as reactants for the production of compounds such asisocyanates.

The term "byproducts" also comprehends aromatic carboxylic acidcompounds containing one, two or three nitro groups and carboxylic acidscontaining one to six carbon atoms, such as formic, acetic or oxalicacid.

These byproducts, a representative number of which having been indicatedabove, are designated either as byproducts or as hydroxynitroaromaticcompounds in the description that follows.

Treatment and/or elimination of the aqueous phase recovered after thevarious washing steps, corresponding to separation of the nitroaromaticcompounds from the byproducts formed, presents a serious disadvantage asthe quantity of these byproducts is considered to be too great.Consequently, the effluents cannot be discharged without priortreatment.

One possible solution to this problem is to collect the effluents andincinerate them, optionally following concentration thereof. However,this technique is not economically viable because the cost of bothconcentrating the effluents and then incinerating them is too high. Inaddition, this technique does not eliminate the problem of dischargingthe inorganic compounds used, particularly for the neutralization step.

Another possible solution is to extract the byproducts, principally thehydroxynitroaromatic compounds, by means of a suitable solvent, thenconcentrating and incinerating the extracted species. However, thisoperation is also of no economic interest because of the multiplicity ofsteps (extraction, concentration) and the requirement for additionalreagents.

Chemical or biochemical treatment of aqueous effluents prior to theirdischarge to destroy byproducts present in the aqueous phase is alsoknown to this art. These methods, however, can also be very costly andentail an extra step in the sequence. In addition, a very large amountof water is discharged, and even if the hydroxynitroaromatic compoundshave been removed, the water nevertheless contains a significantfraction of neutralizing agent which is lost and which should bedestroyed before discharging the effluent to waste.

It will thus be seen that no efficient and economically advantageoustreatment currently exists to combat the problem of waste wateremanating from an aromatic compound nitration process.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofan improved process for the treatment/removal of contaminated wastewateremanating from the synthesis of nitroaromatic compounds, and whichcombines efficiency, simplicity of operation and minimal cost.

Briefly, the present invention features the treatment of aqueouseffluents containing hydroxynitroaromatic compounds, in which a mixtureof nitroaromatic compounds and hydroxynitroaromatic compounds isintimately contacted with a washing medium containing water and aneutralizing agent, then separating the resulting organic and aqueousphases, and further wherein (i) said washing medium is partiallyconstituted by the separated aqueous phase which is recycled, and (ii) afraction of the washing medium is periodically drained for destruction.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THEINVENTION

More particularly according to the present invention, the subjectprocess presents a number of advantages. Aqueous effluent does not haveto be discharged and this is achieved without the requirement for addedprocess steps or for additional reagents, such as solvents or agentswhich destroy or consume the byproducts. In addition, recycling the washwater from the reaction medium considerably reduces neutralizing agentlosses and makes destruction (for example by incineration) or finaldischarge of effluent very attractive economically.

In one embodiment of the invention, the pH of the aqueous phase rangesfrom 5 to 8.

Preferably, the pH of the aqueous phase ranges from 6 to 8.

It should be noted that operating under conditions in which the pH ofthe aqueous phase exceeds the value indicated is possible, even thoughthis is of no particular advantage.

In a first embodiment of the process of the invention, the mixture to betreated may also contain a cosolvent for the nitroaromatic compoundsproduced.

Exemplary such cosolvents include C₆ -C₁₀ aromatic compounds, which maybe substituted by hydrocarbon radicals and/or halogenated. Specificexamples thereof are toluene, cumene and chlorobenzene.

In a second embodiment of the invention, the mixture to be treated doesnot contain any cosolvent.

The operation can be carried out in one or more steps, with stirring.

As is conventional, the mixture and washing medium are contacted at atemperature ranging from 20° C. to 90° C. In the particular event wherethe mixture does not contain a cosolvent, the temperature preferablyranges from 60° C. to 80° C.

This operation is typically carried out at a pressure ranging from 0.5to 10 bar. Advantageously, it is close to atmospheric pressure.

The contact time in each step is advantageously on the order of severalminutes to 2 hours.

Following contacting of the mixture to be treated with the washingmedium, the resulting admixture is decanted and the organic phasecontaining the purified nitroaromatic compounds is separated from theaqueous phase containing the byproducts in the form of salts thereof.

The process of the invention may include one or more washes.

The essential characteristic of the invention is that the washing mediumis partially constituted by the recovered aqueous phase which isrecycled to the washing process. In addition, a fraction of said washingmedium is periodically drained before destruction (incineration,discharge).

In a preferred embodiment of the invention, the washing medium isrecycled such that the concentration of byproducts is increased by afactor of 10 with reference to the amount present in the aqueous phasefrom the first wash. Recycling is preferably carried out such that theconcentration of byproducts is increased by a factor of at least 20, upto a factor of 30.

It should be appreciated that the process can be operated underconditions where the concentration is outside the values indicatedabove. However, if the concentration is increased by a factor of lessthan 10, the advantages presented by recycling are less marked.Similarly, a concentration of more than 30 times the originalconcentration risks polluting the organic phase by transfer ofbyproducts to the organic phase, thus presenting difficulties during thehydrogenation reaction.

The drained fraction advantageously represents 3% to 10% by weight ofthe total amount of washing medium.

Since the process of the invention is preferably carried outcontinuously, draining a fraction of the washing medium necessitatesadding an amount of washing medium corresponding to the quantitydrained. It will be appreciated that this constitutes an additionaladvantage of the subject process. Indeed, contrary to conventionalprocesses which do not include a recycling step, the amount of washingmedium introduced into the sequence is considerably less than hatnormally used (3% to 10%).

This addition can be carried out at any situs in the process zone wherethe mixture of nitroaromatic compounds and byproducts is washed.

The washing apparatus is conventional and one skilled in this art coulduse one or more storage reservoirs for the washing medium, positionedbetween two washing tanks.

Following the washing step(s), the organic phase, which has beenseparated from the aqueous phase, can if necessary be washed with waterto recover all traces of neutralizing agent.

The resulting effluent, which may contain trace amounts of neutralizingagent, can be discharged, if necessary following a purificationtreatment.

This washing water, however, is advantageously recycled to the processof the invention. In particular, the stream can be reintroduced to thefirst washing step which eliminates the mineral acids from the nitrationreaction products.

The process of the invention is normally carried out with stirring at atemperature ranging from 20° C. to 90° C., for a period of time ofseveral minutes to 2 hours.

The resulting mixture is then decanted and the organic phase, containingthe nitroaromatic compounds, is separated from the aqueous phase.

Said purified nitroaromatic compounds can then be used for thepreparation of aromatic diamines.

The nitroaromatic compounds, if necessary after drying by any knownmeans, are then reacted with pure or diluted hydrogen in the presence ofa conventional hydrogenation catalyst, for example Raney nickel or aplatinum based catalyst.

The hydrogenation reaction temperature generally ranges from 80° C. to200° C. and the pressure during the reaction advantageously ranges from1 to 150 bar.

In order to further illustrate the present invention and the advantagesthereof, the following specific example is given, it being understoodthat same is intended only as illustrative and in nowise limitative.

EXAMPLE

A dilute aqueous ammonia solution and a mixture of dinitrotoluenescontaining trinitrocresols were introduced, in parallel, into a stirredcontinuous reactor communicating with a decanter positioned downstreamthereof.

The temperature was 70° C. and the residence time in the washing anddecanting zone was one hour.

The dinitrotoluenes had already been washed in a first washing step toeliminate the mineral acids present during the nitration reaction

The amount of organic acid species in this mixture, principallycontaining nitrocresols, was 1,300 ppm.

The dinitrotoluene and dilute ammonia flow rates were 500 g/h and 150g/h, respectively.

A minimum of 90% of the aqueous phase from the decanter was recycled andreintroduced into the reactor at a recycle flow rate of 135 g/h.

Once recycling had commenced, the ammonia flow rate was reduced to 10%of the total flow rate, i.e., 15 g/h. This addition was such that the pHof the aqueous phase was maintained at a constant value within the range7-8.

The added neutralizing agent was continuously drained for incineration.

The concentration of byproducts (nitrocresols, nitrophenols,nitrobenzoic acids) and that of the inorganic species (ammonium ionsfrom the neutralizing agent, nitrates, residual sulfates not extractedduring the first washing step) was increased via this technique.

A 1% concentration of byproducts was attained in this instance.

The nitrotoluenes obtained were washed with water at 70° C. in a secondreactor, also communicating with a downstream decanter, and were thentransferred to another reactor for hydrogenation.

A hydrogenation test was carried out on the resulting nitrotoluenes toprovide quality control.

This test entailed introducing the nitrotoluenes into a stirred reactorcontaining isopropanol and Raney nickel at a temperature of 130° C.,under a hydrogen atmosphere and at a total pressure of 35 bar.

The degree of hydrogenation was obtained by measuring the pressure ofhydrogen which had to be introduced into the reactor to maintain thetotal pressure at 35 bar.

This test evidenced that the degree or extent of hydrogenation of thenitrotoluenes obtained employing the process of the invention wasidentical to that of the nitrotoluenes obtained employing the samewashing steps, but without recycling.

Thus, the process of this invention did not adversely affect the qualityof the nitrotoluenes produced.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for the treatment/removal of byproductaqueous effluent containing contaminating amounts ofhydroxynitroaromatic compounds, comprising (a) intimately contacting amixture of at least one nitroaromatic compound and at least onehydroxynitroaromatic compound with an aqueous wash medium whichcomprises a neutralizing agent, (b) separating the resulting admixtureinto an organic phase and an aqueous phase, (c) recycling a fraction ofsaid separated aqueous phase to said aqueous wash medium, (d)periodically draining a fraction of said aqueous wash medium, (e) addinga replenishing amount of aqueous wash medium corresponding to thedrained fraction thereof, and (f) destroying the drained fraction of theaqueous wash medium without further treatment.
 2. The process as definedby claim 1, comprising (d) periodically draining from 3% to 10% of thetotal amount of said aqueous wash medium.
 3. The process as defined byclaim 1, said neutralizing agent comprising a reactant that convertssaid at least one hydroxynitroaromatic compound into a water solublesalt thereof.
 4. The process as defined by claim 3, said neutralizingagent comprising an alkali and/or alkaline earth metal hydroxide,carbonate or bicarbonate, or ammonia.
 5. The process as defined by claim1, wherein the concentration of said neutralizing agent in said aqueouswash medium ranges from 0.1% to 50% by weight.
 6. The process as definedby claim 5, said concentration ranging from 2% to 20% by weight.
 7. Theprocess as defined by claim 1, wherein the pH during intimate contactingof said nitroaromatic/hydroxynitroaromatic compound mixture with saidaqueous wash medium ranges from 5 to
 8. 8. The process as defined byclaim 7, said pH ranging from 6 to
 8. 9. The process as defined by claim1, said intimate contacting being at a temperature ranging from 20° C.to 90° C.
 10. The process as defined by claim 9, said temperatureranging from 60° C. to 80° C.
 11. The process as defined by claim 1,said at least one nitroaromatic compound comprising a nitro- orpolynitrotoluene.
 12. The process as defined by claim 1, said at leastone hydroxynitroaromatic compound comprising a nitro- orpolynitrophenol, a nitro- or polynitrocresol, a nitro- orpolynitroaromatic carboxylic acid, or mixture thereof.
 13. The processas defined by claim 1, further comprising recovering said at least onenitroaromatic compound from said separated organic phase.
 14. Theprocess as defined by claim 1, comprising a plurality ofcontacting/separating sequences.